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We have investigated hybridized magnon-phonon excitation (magnon polarons) in spin Seebeck effects (SSEs) in Bi$_x$Y$_{3-x}$Fe$_{5}$O$_{12}$ (Bi$_x$Y$_{3-x}$IG; $x=0$, $0.5$, and $0.9$) films with Pt contact. We observed sharp peak structures in the magnetic field $H$ dependence of the longitudinal SSE (LSSE) voltages, which appear when the phonon dispersions are tangential to the magnon dispersion curve in Bi$_x$Y$_{3-x}$IG. By increasing the Bi amount $x$, the peak fields in the LSSE shift toward lower $H$ values due to the reduction of the sound velocities in Bi$_x$Y$_{3-x}$IG. We also measured the SSE in a nonlocal configuration and found that magnon-polaron anomalies appear with different signs and intensities. Our result shows composition-tunability of magnon-polaron anomalies and provides a clue to further unravel the physics of magnon-polaron SSEs., Comment: Editors' Suggestion, 14 pages, 11 figures

We have theoretically investigated the spin Seebeck effect (SSE) in a normal metal (NM)/paramagnetic insulator (PI) bilayer system. Through a linear response approach, we calculated the thermal spin pumping from PI to NM and backflow spin current from NM to PI, where the spin-flip scattering via the interfacial exchange coupling between conduction-electron spin in NM and localized spin in PI is taken into account. We found a finite spin current appears at the interface under the difference in the effective temperatures between spins in NM and PI, and its intensity increases by increasing the density of the localized spin $S$. Our model well reproduces the magnetic-field-induced reduction of the paramagnetic SSE in Pt/Gd$_3$Ga$_5$O$_{12}$ experimentally observed when the Zeeman energy is comparable to the thermal energy, which can be interpreted as the suppression of the interfacial spin-flip scattering. The present finding provides an insight into the mechanism of paramagnetic SSEs and the thermally induced spin-current generation in magnetic materials., Comment: 8 page, 5 figures

We have investigated the anomalous Ettingshausen effect (AEE) in iron-carbon alloys, i.e., cast irons and steel, using the lock-in thermography. All the alloys exhibit the clear AEE-induced temperature modulation, and their anomalous Ettingshausen coefficient is an order of magnitude greater than that of the pure iron at room temperature. The dimensionless figure of merit for AEE in the ductile cast iron is 55 times greater than that in the pure iron owing to the significant increase of the anomalous Ettingshausen coefficient. Our result reveals a potential of iron-carbon alloys as transverse thermoelectric materials, although the composition and microstructures optimizations are necessary., Comment: 15 pages, 4 figures

When the electric conductance of a nano-sized metal is measured at low temperatures, it often exhibits complex but reproducible patterns as a function of external magnetic fields, called quantum fingerprints in electric conductance. Such complex patterns are due to quantum-mechanical interference of conduction electrons; when thermal disturbance is feeble and coherence of the electrons extends all over the sample, the quantum interference pattern reflects microscopic structures, such as crystalline defects and the shape of the sample, giving rise to complicated interference. Although the interference pattern carries such microscopic information, it looks so random that it has not been analysed. Here we show that machine learning allows us to decipher quantum fingerprints; fingerprint patterns in magneto-conductance are shown to be transcribed into spatial images of electron wave function intensities (WIs) in a sample by using generative machine learning. The output WIs reveal quantum interference states of conduction electrons, as well as sample shapes. The present result augments the human ability to identify quantum states, and it should allow microscopy of quantum nanostructures in materials by making use of quantum fingerprints., Comment: This is a post-peer-review, pre-copyedit version of an article published in Nature Communications. The final authenticated version is available online at: http://dx.doi.org/10.1038/s41467-022-30767-w

A triplon refers to a fictitious particle that carries angular momentum $S = 1$ corresponding to the elementary excitation in a broad class of quantum dimerized spin systems. Such systems without magnetic order have long been studied as a testing ground for quantum properties of spins. Although triplons have been found to play a central role in thermal and magnetic properties in dimerized magnets with singlet correlation, a spin angular momentum flow carried by triplons, a triplon current, has not been detected yet. Here we report spin Seebeck effects induced by a triplon current: triplon spin Seebeck effect, using a spin-Peierls system CuGeO$_3$. The result shows that the heating-driven triplon transport induces spin current whose sign is positive, opposite to the spin-wave cases in magnets. The triplon spin Seebeck effect persists far below the spin-Peierls transition temperature, being consistent with a theoretical calculation for triplon spin Seebeck effects.

Spin Hall magnetoresistance (SMR) refers to a resistance change in a metallic film reflecting the magnetization direction of a magnet attached to the film. The mechanism of this phenomenon is spin exchange between conduction-electron spins and magnetization at the interface. SMR has been used to read out information written in a small magnet and to detect magnetization dynamics, but it has been limited to magnets; magnetic ordered phases or instability of magnetic phase transition has been believed to be indispensable. Here, we report the observation of SMR in a paramagnetic insulator Gd$_{3}$Ga$_{5}$O$_{12}$ (GGG) without spontaneous magnetization combined with a Pt film. The paramagnetic SMR can be attributed to spin-transfer torque acting on localized spins in GGG. We determine the efficiencies of spin torque and spin-flip scattering at the Pt/GGG interface, and demonstrate these quantities can be tuned with external magnetic fields. The results clarify the mechanism of spin-transport at a metal/paramagnetic insulator interface, which gives new insight into the spintronic manipulation of spin states in paramagnetic systems., Comment: 40 pages, 15 figures

We report the observation of anomalous peak structures induced by hybridized magnon-phonon excitation (magnon polarons) in the magnetic field dependence of the spin Peltier effect (SPE) in a Lu$_{2}$Bi$_{1}$Fe$_{4}$Ga$_{1}$O$_{12}$ (BiGa:LuIG) with Pt contact. The SPE peaks coincide with magnetic fields tuned to the threshold of magnon-polaron formation, consistent with the previous observation in the spin Seebeck effect. The enhancement of SPE is attributed to the lifetime increase in spin current caused by magnon-phonon hybridization in BiGa:LuIG., Comment: Phys. Rev. B (in press), 7 pages, 5 figures

Long-distance transport of spin information in insulators without long-range magnetic order has been recently reported. Here, we perform a complete characterization of amorphous Y$_3$Fe$_5$O$_{12}$ (a-YIG) films grown on top of SiO$_2$. We confirm a clear amorphous structure and paramagnetic behavior of our a-YIG films, with semiconducting behavior resistivity that strongly decays with increasing temperature. The non-local transport measurements show a signal which is not compatible with spin transport and can be attributed to the drop of the a-YIG resistivity caused by Joule heating. Our results emphasize that exploring spin transport in amorphous materials requires careful procedures in order to exclude the charge contribution from the spin transport signals., Comment: 10 pages, 4 figures

We report the nonlocal spin Seebeck effect (nlSSE) in a lateral configuration of Pt/Y$_3$Fe$_5$O$_{12}$(YIG)/Pt systems as a function of the magnetic field $B$ (up to 10 T) at various temperatures $T$ (3 K < $T$ < 300 K). The nlSSE voltage decreases with increasing $B$ in a linear regime with respect to the input power (the applied charge-current squared $I^2$). The reduction of the nlSSE becomes substantial when the Zeeman energy exceeds thermal energy at low temperatures, which can be interpreted as freeze-out of magnons relevant for the nlSSE. Furthermore, we found the non-linear power dependence of the nlSSE with increasing $I$ at low temperatures ($T$ < 20 K), at which the $B$-induced signal reduction becomes less visible. Our experimental results suggest that in the non-linear regime high-energy magnons are over populated than those expected from the thermal energy. We also estimate the magnon spin diffusion length as functions of $B$ and $T$., Comment: 4 pages

The discovery of new materials that efficiently transmit spin currents has been important for spintronics and material science. The electric insulator $\mathrm{Gd}_3\mathrm{Ga}_5\mathrm{O}_{12}$ (GGG) is a superior substrate for growing magnetic films, but has never been considered as a conduit for spin currents. Here we report spin current propagation in paramagnetic GGG over several microns. Surprisingly, the spin transport persists up to temperatures of 100 K $\gg$ $T_{\mathrm{g}} = 180$ mK, GGG's magnetic glass-like transition temperature. At 5 K we find a spin diffusion length ${\lambda_{\mathrm{GGG}}} = 1.8 \pm 0.2 {\mu}$m and a spin conductivity ${\sigma}_{\mathrm{GGG}} = (7.3 \pm 0.3) \times10^4$ $\mathrm{Sm}^{-1}$ that is larger than that of the record quality magnet $\mathrm{Y}_3\mathrm{Fe}_5\mathrm{O}_{12}$ (YIG). We conclude that exchange coupling is not required for efficient spin transport, which challenges conventional models and provides new material-design strategies for spintronic devices., Comment: 21 pages, 4 figures

We report the observation of magnetic-field-induced suppression of the spin Peltier effect (SPE) in a junction of a paramagnetic metal Pt and a ferrimagnetic insulator ${\rm Y_{3}Fe_{5}O_{12}}$ (YIG) at room temperature. For driving the SPE, spin currents are generated via the spin Hall effect from applied charge currents in the Pt layer, and injected into the adjacent thick YIG film. The resultant temperature modulation is detected by a commonly-used thermocouple attached to the Pt/YIG junction. The output of the thermocouple shows sign reversal when the magnetization is reversed and linearly increases with the applied current, demonstrating the detection of the SPE signal. We found that the SPE signal decreases with the magnetic field. The observed suppression rate was found to be comparable to that of the spin Seebeck effect (SSE), suggesting the dominant and similar contribution of the low-energy magnons in the SPE as in the SSE.

We have fabricated ferrite cantilevers in which their vibrational properties can be controlled by external magnetic fields. Submicron-scale cantilever structures were made from Y3Fe5O12 (YIG) films by physical etching combined with use of a focused ion beam milling technique. We found that the cantilevers exhibit two resonance modes which correspond to horizontal and vertical vibrations. Under external magnetic fields, the resonance frequency of the horizontal mode increases, while that of the vertical mode decreases, quantitatively consistent with our numerical simulation for magnetic forces. The changes in resonance frequencies with magnetic fields reach a few percent, showing that efficient magnetic control of resonance frequencies was achieved., Comment: Appl. Phys. Lett